but amd64, it just returns 0, doing nothing.
For amd64, it implements vsyscalls through cheating: if the faulting
address is in the vsyscall area (which is statically known on Linux/amd64),
and the intruction pointer is too, it must have been a vsyscall. In that
case, retrieve the return address from the user stack, fix up %rip and
%rsp, and just execute the normal system call. It will return as if
the vsyscall has been executed.
33e2d79f47